Even cheap triple-axis magnetometer ICs, like those used in some smartphones could give interesting data if you put them in stable temperature conditions. Lot of such modules on aliexpress for a buck, not very sensitive, but definitely enough to detect serious disturbances in Earth magnetic field.
Didn't find any signs of weakening Earth magnetic field.
https://www.spaceweatherlive.com/en/auroral-activity/magnetometers.html
There is DIY Earth field magnetometers projects like http://avtanski.net/projects/magnetometer/design.html , pretty interesting thing to build, may be I will spend some weekend on it.
Even cheap triple-axis magnetometer ICs, like those used in some smartphones could give interesting data if you put them in stable temperature conditions. Lot of such modules on aliexpress for a buck, not very sensitive, but definitely enough to detect serious disturbances in Earth magnetic field.